Why digital chalk and pastel look different from a brush

The short answer

A brush lays down a continuous ribbon of colour along a path. A stick of chalk, charcoal or pastel does not lay anything down — it is abraded by the surface, and what you see is loose grain caught in the texture. Those two physical processes produce different pictures, and a drawing app that models them the same way will always make chalk look like a grey brush with noise sprinkled on top.

Stamping versus stroking

How a brush tool is usually built

The straightforward way to draw a stroke is to take the path your hand made and render a smooth outline along it, widened by pressure. The result has clean mathematical edges, and colour inside the outline is even. That is a good model for ink and paint, and a bad one for anything dry.

How dry media have to be built

Flying Chalk never renders a path. It walks along the route your hand took and stamps a grainy mark at regular intervals, each one rotated and nudged by a small random amount. Everything that makes chalk look like chalk falls out of that choice: the edge of a line is ragged because it is the union of many irregular stamps, the interior has visible tooth because the stamps do not perfectly overlap, and two strokes crossing look like dust sitting on dust rather than one flat colour covering another.

Why speed matters more than it does with a brush

Move a real stick fast and it skips: the mark goes thin, breaks up, and leaves gaps where the stick failed to bite. Move it slowly and the powder piles up until the line is solid. Because a stamping engine places its marks at intervals along the path, this behaviour comes almost for free — fast movement spreads the stamps apart and lightens them, slow movement packs them until they merge. With a brush model, speed can only ever change the width, which is why brush-simulated chalk never quite skips.

The scaling trap

There is a subtle engineering consequence worth knowing about if you zoom in while drawing. If an app measures "fast" in screen pixels, then at high zoom the canvas barely moves per sample, the engine concludes that you are always drawing slowly, and the skipping disappears exactly when you are working on fine detail. The fix is to measure speed relative to the current stroke width rather than in absolute pixels, so that the same hand movement means the same thing at any zoom level. Flying Chalk does it that way, which is why the grain still behaves when you pinch in.

The surface is half the result

Dry media are defined by what they are dragged across. The same stick reads completely differently on a slate board, on kraft paper, on Xuan paper and on smooth card, because the tooth of the surface decides how much grain is caught and where. This is why a chalk app needs real surfaces with their own noise rather than a flat coloured background — a flat fill gives the grain nothing to sit in, and the stroke ends up floating on top of the page. Flying Chalk ships eleven surfaces, including grid, ruled, Tian Zi Ge and a music staff, each with its own texture rather than a plain colour.

Why each dry tool still has to differ

Chalk, crayon, oil pastel, charcoal and pencil are all "grain on a surface", but they are not interchangeable. Oil pastel spreads and broadens far more than chalk when you lean on it. Charcoal stretches the furthest on its side and gets darker. Graphite on its side goes paler and greyer. In Flying Chalk each of the eight sticks carries its own grain, spacing and tilt numbers, so switching tool changes behaviour rather than just changing colour.

The bonus you get from stamping

Because a stroke is stored as points plus a random seed rather than as pixels, the whole drawing can be replayed rather than snapshotted. Undo five times and redo five times, and every speck of grain lands exactly where it was — no shimmer, no re-rolled texture. A pixel-based canvas cannot promise that without storing a full image for every step.

Related guides

• Apple Pencil tilt shading: how it works and how to use it
• Drawing on an iPad with your finger vs an Apple Pencil
• Apple Pencil pressure vs tilt vs azimuth, explained